Referring to U.S. Pat. No. 5,088,778 and FIGS. 11 to 13, a conventional transmission mechanism 400 of a handle lock set comprises a driving plate 410, a transmission plate 420 and a cover 430, wherein an end 411 of the driving plate 410 is used to push a cylinder 421 of the transmission plate 420 for moving upwardly. With reference to FIG. 12, when the transmission plate 420 reaches a transmission dead spot (that means the transmission plate 420 is in contact with top of the cover 430) in moving process, the driving plate 410 likely falls off from curved surface of the cylinder 421 as illustrated in FIG. 13 when user apply more powerful forces to rotate the driving plate 410 because the end 411 of the driving plate 410 contacts with curved surface of the cylinder 421 of the transmission plate 420. Thus, the driving plate 410 is not able to be pushed to return to original position by the transmission plate 420 and makes the transmission mechanism 400 of the handle lock set out of use.